Science STD 7 Chapter 11: Light - Exercises
1. Fill in the blanks:
- (a) An image that cannot be obtained on a screen is called virtual image.
- (b) Image formed by a convex mirror is always virtual and smaller in size.
- (c) An image formed by a plane mirror is always of the same size as that of the object.
- (d) An image which can be obtained on a screen is called a real image.
- (e) An image formed by a concave lens cannot be obtained on a screen.
2. Mark ‘T’ if the statement is true and ‘F’ if it is false:
- (a) We can obtain an enlarged and erect image by a convex mirror. (T/F)
F
- (b) A concave lens always form a virtual image. (T/F)
T
- (c) We can obtain a real, enlarged and inverted image by a concave mirror. (T/F)
T
- (d) A real image cannot be obtained on a screen. (T/F)
F
- (e) A concave mirror always form a real image. (T/F)
F
3. Match the items given in Column I with one or more items of Column II.
| Column I | Column II |
|---|---|
| (a) A plane mirror | (v) The image is erect and of the same size as the object. |
| (b) A convex mirror | (ii) Can form image of objects spread over a large area. |
| (c) A convex lens | (i) Used as a magnifying glass. |
| (d) A concave mirror | (iii) Used by dentists to see enlarged image of teeth. |
| (e) A concave lens | (vi) The image is erect and smaller in size than the object. |
4. State the characteristics of the image formed by a plane mirror.
The image formed by a plane mirror is:
- It is **virtual**, meaning it cannot be obtained on a screen.
- It is **erect** or upright.
- It is of the **same size** as the object.
- It is formed at the **same distance** behind the mirror as the object is in front of it.
- It is **laterally inverted**, meaning the left side of the object appears on the right side in the image, and vice-versa.
5. Find out the letters of English alphabet or any other language known to you in which the image formed in a plane mirror appears exactly like the letter itself. Discuss your findings.
The English letters in which the image formed in a plane mirror appears exactly like the letter itself are those that are symmetrical along a vertical axis. These letters are: **A, H, I, M, O, T, U, V, W, X, Y**. For example, the letter 'A' looks the same in a mirror because it is symmetrical, whereas 'F' is not and appears laterally inverted.
6. What is a virtual image? Give one situation where a virtual image is formed.
A **virtual image** is an image that cannot be obtained on a screen. The light rays appear to diverge from the image but do not actually meet. A virtual image is formed in a **plane mirror**, such as when you look at yourself in a bathroom mirror. The image you see is virtual because it is formed behind the mirror and cannot be projected onto a screen.
7. State two differences between a convex and a concave lens.
| Convex Lens | Concave Lens |
|---|---|
| It is thicker in the middle and thinner at the edges. | It is thinner in the middle and thicker at the edges. |
| It converges (bends inwards) light rays. | It diverges (bends outwards) light rays. |
| It can form both real and virtual images. | It always forms a virtual image. |
8. Give one use each of a concave and a convex mirror.
- Concave mirror: Used by dentists to see an enlarged image of teeth.
- Convex mirror: Used as side-view mirrors in cars and scooters to see a wide area of traffic behind.
9. Which type of mirror can form a real image?
A **concave mirror** can form a real image. A real image is an image that can be obtained on a screen.
10. Which type of lens forms always a virtual image?
A **concave lens** always forms a virtual image.
11. A virtual image larger than the object can be produced by a
- concave lens
- concave mirror
- convex mirror
- plane mirror
12. David is observing his image in a plane mirror. The distance between the mirror and his image is 4 m. If he moves 1 m towards the mirror, then the distance between David and his image will be
When David moves 1 m towards the mirror, the distance between him and the mirror becomes 3 m. In a plane mirror, the image is formed at the same distance behind the mirror. So, the distance of the image from the mirror will also be 3 m. The total distance between David and his image will be 3 m + 3 m = **6 m**.
- 3 m
- 5 m
- 6 m
- 8 m
13. The rear view mirror of a car is a plane mirror. A driver is reversing his car at a speed of 2 m/s. The driver sees in his rear view mirror the image of a truck parked behind his car. The speed at which the image of the truck appears to approach the driver will be
A plane mirror forms an image that moves at the same speed as the object relative to the mirror. Since the driver is reversing at 2 m/s, his car is moving towards the mirror at this speed. The image of the parked truck will also appear to move towards the driver at a speed of 2 m/s. However, the driver observes his own image also moving towards him at 2 m/s. The relative speed at which the image of the truck appears to approach the driver is the sum of these speeds, i.e., 2 m/s (driver's speed) + 2 m/s (image's speed) = 4 m/s.
- 1 m/s
- 2 m/s
- 4 m/s
- 8 m/s
Suggested Activities and Projects
1. Play with a mirror. Write your name with a sketch pen on a thin sheet of paper, polythene or glass. Read your name on the sheet while standing in front of a plane mirror. Now look at your image in the mirror.
Outline: This is a simple activity to demonstrate **lateral inversion**. By writing your name and observing it in a mirror, you will see that the letters appear reversed, or flipped horizontally. This happens because a plane mirror inverts the image from left to right, but not upside down.
2. A burning candle in water. Take a shoe box, open on one side. Place a small lighted candle in it. Place a clear glass sheet (roughly 25 cm × 25 cm) infront of this candle (Fig. 11.33). Try to locate the image of the candle behind the glass sheet. Place a glass of water at its position. Ask your friends to look at the image of the candle through the sheet of glass. Ensure that candle is not visible to your friends. Your friends will be surprised to see the candle burning in water. Try to explain the reason.
Outline: This activity is a magic trick that uses reflection. The glass sheet acts as a mirror, and the burning candle's image is formed behind it. When you place a glass of water at the exact position of the image, it looks as if the candle is burning inside the water. The actual candle is hidden, and the friends only see the reflected image. This shows how a clear glass sheet can act as a partial reflector.
3. Make a rainbow. Try to make your own rainbow. You can try this project in the morning or in the evening. Stand with your back towards the Sun. Take a hosepipe or a water pipe used in the garden. Make a fine spray in front of you. You can see different colours of rainbow in the spray.
Outline: This project demonstrates how sunlight is composed of seven colours. The tiny water droplets in the fine spray act like small prisms. When sunlight passes through them, it splits into its seven constituent colours (red, orange, yellow, green, blue, indigo, violet), forming a rainbow. For this to work, you must stand with your back to the sun, and the spray must be in front of you.
4. Visit a laughing gallery in some science centre or a science park or a village mela. You will find